VIBRATIONAL CIRCULAR DICHROISM STUDIES OF EPHEDRINE AND RELATED MOLECULES

Abstract

Vibrational circular dichroism spectra of $(1R,2S)$-norephedrine, $(1R,2R)$-pseudoephedrine and $(1R,2R)$-norpseudoephedrine in the CH-stretching region will be presented. The most abundant conformations of these pharinacologically active molecules in $CDCl_{3}$ and in $D_{2}O$ solution at high and low pH can be deduced from the sign and magnitude of the methine stretching VCD bands for chiral centers 1 and 2. The interpretation is based on application of Rule 1 of the ring current mechanism, due to current in OH--N or NH--O intramolecularly hydrogen-bonded rings. The VCD spectrum of norephedrine is consistent with only one of the two gauche conformations indistinguishable by NMR. The trans conformer is preferred in norpseudoephedrine.